This invention relates to the art of thermal recording using a thermal head, which is applicable to various types of printers, plotters, facsimile, recorders, etc.
Thermal materials comprising a thermal recording layer unit on a substrate of a film or the like are commonly used to record images produced in diagnosis by ultrasonic scanning.
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a glaze in which heating elements comprising heaters and electrodes, used for heating the thermal recording layer unit of a thermal material to record an image are arranged in one direction (main scanning direction) and, with the glaze urged at small pressure against the thermal material (thermal recording layer unit), the two members are moved relative to each other in the auxiliary scanning direction perpendicular to the main scanning direction, and the heaters of the respective pixels on the glaze are heated by energy application in accordance with image data to be recorded which were supplied from an image data supply source such as MRI in order to heat the thermal recording layer unit of the thermal material, thereby accomplishing image reproduction.
The glaze of the thermal head has a protective layer formed on the surface in order to protect the heaters for heating thermal materials, the associated electrodes and the like. It is this protective layer that contacts the thermal material during thermal recording and the heaters heat the thermal material through this protective layer so as to perform thermal recording.
The protective layer of the thermal head is usually made of wear-resistant ceramics; however, during thermal recording, the surface of the protective layer is heated and kept in sliding contact with the thermal material, so it will gradually wear and deteriorate upon repeated recording.
If the wear of the protective layer of the thermal head progresses, density nonuniformity or density variations will occur in the thermal image or the desired protective strength can not be maintained and, hence, the ability of the layer to protect the heaters is impaired to such an extent that the intended image recording is no longer possible (the head has lost its function). Particularly in the application such as the aforementioned medical use which requires variable-contrast images of high quality, the trend is toward ensuring the desired high image quality by adopting thermal films with highly rigid substrates such as polyester films and also increasing the settings of recording temperature (energy applied) and of the pressure at which the thermal head is urged against the thermal material.
Under these circumstances, as compared with the conventional thermal recording, a greater force (mechanical stress) and more heat are exerted on the protective layer of the thermal head, making wear and corrosion (or wear due to corrosion) more likely to progress. What is more, in thermal films using polyester films or like substrates, water and other substances in the thermal recording layer unit which will cause corrosion do not penetrate into the substrate but deposit on the surface, or the protective layer, of the thermal head; this tends to increase the concentrations of the corrosive substances in the surface of the protective layer, also resulting in accelerated corrosion.
With a view to preventing the wear of the protective layer on the thermal head so as to improve its durability, various methods have been proposed and commercialized on the basis of improvements in different aspects such as the protective layer per se, the thermal material and the recording conditions. Among these methods, the improvement of the thermal material is primarily based on the reduction of components that will cause wear and corrosion; however, this approach occasionally involves side effects such as head stain and sticking which make it difficult to achieve the intended anti-wear effect. The improvement of the recording conditions is based on the reduction of the maximum temperature that is employed by thermal recording, as well as the recording pressure; a problem with this approach is that the quality of the image being recorded is sometimes affected and the intended effectiveness is not attained in applications that require high image quality.
In order to prevent the wear of the protective layer on the thermal head, many techniques for improving its performance have heretofore been reviewed. To this end, attempts have been made to improve the protective layer and various proposals made. For example, SIALON (Si--Al--O--N) is known as a material suitable for use in the protective layer on the thermal head. According to Unexamined Published Japanese Patent Application (KOKAI) No. 193171/1993, a method is disclosed for improving the wear resistance of the protective layer on the thermal head by adjusting the relative proportions of the components of SIALON (Si--Al.sub.x --O.sub.y --N.sub.z) to x=0.1-4.0, y=0.2-6.0 and z=1.2-3.0. Unexamined Published Japanese Patent Application (KOKAI) No. 216762/1988 discloses a method of preventing the delamination of the protective layer on the thermal head by adding a metal to SIALON; Unexamined Published Japanese Patent Application (KOKAI) No. 8501/1994 discloses a method of adjusting the hardness and electrical resistance of SIALON by addition of titanium nitride (TiN); and Unexamined Published Japanese Patent Application (KOKAI) No. 31961/1994 discloses a method of adjusting the thermal expansion of SIALON by addition of titanium boride (TiB)
Thin films of metal oxides such as chromium oxide (CrO) are also used as the protective layer of the thermal head and Unexamined Published Japanese Patent Application (KOKAI) No. 305720/1993 discloses a method in which the wear resistance of a protective layer made of chromium oxide is improved by addition of at least one oxide selected from the group consisting of silicon oxide, titanium oxide, aluminum oxide, cerium oxide and yttrium oxide or by addition of a nitride of a high-melting point metal (Cr or Ti).
According to these methods, the characteristics of the protective layer, particularly its wear resistance, are sufficiently improved to realize satisfactory thermal heads; however, under high-temperature, high-pressure recording conditions which are intended to record high-quality images, it is still impossible to provide sufficient wear resistance to ensure adequate reliability for the protective layer and various troubles including the functional failure of the head (breakage of heaters) and uneven uniformity in the recorded image will occur on account of the worn protective layer.
Other methods have been proposed to improve the wear resistance of the protective layer on the thermal head; for example, Unexamined Published Japanese Patent Application (KOKAI) No. 227763/1987 discloses the use of a thin diamond film as the protective layer, and Unexamined Published Japanese Patent Application (KOKAI) No. 72559/1988 discloses the use of a thin film solely made of or based on carbon and which has a thermal conductivity of at least 2.5 W/cm.multidot.deg as the protective layer. These methods are capable of forming protective layers that are very hard to feature high wear resistance; on the other hand, the protective layers thus formed have such a low electrical insulating property that on account of the static electricity generated by sliding contact with the thermal material, the heaters and the electrodes are prone to break.
Further, it has been proposed that the wear resistance of the protective layer on the thermal head be improved by composing it of more than one sub-layer so as to realize a thermal head having improved durability. For example, Unexamined Published Japanese Patent Application (KOKAI) No. 189958/1986 discloses a thermal head which has a protective layer comprising a first sub-layer made of a heat-resistant material having a volume resistivity of 10.sup.3 -10.sup.13 .OMEGA..multidot.cm and an overlying second sub-layer made of superhard carbon, whereby not only the resistance of the protective layer to wear and environment is improved, but also high thermal efficiency is assured. In addition, Unexamined Published Japanese Patent Application (KOKAI) No. 132628/1995 discloses a thermal head which has a dual protective layer comprising a silicon-based compound sub-layer and an overlying diamond-like carbon sub-layer, whereby the potential wear and breakage of the protective layer are significantly reduced to ensure that high-quality image can be recorded over an extended period of time.
These dual protective layers have satisfactory wear resistance under mechanical stress but they often fail to exhibit high enough endurance against the wear caused by corrosive substances in the thermal recording material. According to the studies conducted by the present inventors, moisture has a particularly great influence on these protective layers and they do not prove fully effective in the process of thermal recording which is performed under high enough energy and pressure conditions to produce high-quality images.